When a website changes to a new server (with a different IP address), or when you register a new domain, nameservers around the world must be notified of the change. This process of notification is referred to as propagation.
Changes to DNS records do not propagate throughout the Internet immediately, as all server caches first need to expire and refresh, which typically takes 24 to 72 hours.
During this time, if you are transferring your domain to another service provider, some nameservers will still refer to your site’s old hosting location while nameservers that have already been notified of the change will refer to the new location. As a result, visitors accessing your site may be directed to either the old or new location, depending upon which nameserver they connect to.